Vietnamese authorities have deported an American pro-democracy activist after detaining him for nine months on charges of attempting to overthrow the state, state media reported on Wednesday.

Nguyen Quoc Quan, who is Vietnamese origin and a member of the US-based Viet Tan Party -- which Hanoi views as a terrorist organisation -- was arrested on April 17 last year when he arrived in Vietnam.

Quan, 59, also known as Richard Nguyen, was due to go on trial earlier this month, Viet Tan told AFP at the time, but the proceedings were cancelled without an official explanation.

“Vietnam has expelled Nguyen Quoc Quan, who is an American citizen,” according to an online report on Vietnam News Agency late Wednesday.

“Quan admitted his crimes, asking for leniency so that he could go back to the United States and be with his family,” the report said, without adding further details.

Quan’s lawyer could not be immediately reached for comment, but Viet Tan spokeswoman Trinh Nguyen confirmed the release on Wednesday and told AFP Quan would soon land at Los Angeles airport.

The US embassy spokesperson in Hanoi, Christopher Hodges, told AFP after the deportation was announced that they were “encouraged” by Quan’s release.

State media had said Quan travelled to the communist country “to instigate a demonstration and undermine celebrations” of the fall of the US-backed southern regime on April 30, which heralded the end of a decades-long conflict.

He was initially charged with terrorism offences, but authorities later proposed that he be tried for plotting to overthrow the state -- a charge that can carry the death penalty, although the communist regime has never executed anyone on political charges.

Quan previously received a six-month jail sentence on terrorism charges and was deported from Vietnam in May 2008.

Rights groups say allegations of spreading anti-state propaganda and attempting to overthrow the regime are routinely laid against peaceful dissidents in a country where the Communist Party forbids political debate.

Dozens of peaceful political activists have been jailed since Vietnam, a one-party state, began a new crackdown on free expression in late 2009.

So far in 2013, 14 activists have been convicted on the same charges in two separate court proceedings.